Pen cap



FIG. 1 is a front elevation view of my design for a pen cap. FIG. 2 is a side elevation view thereof. FIG. 3 is a top view thereof; and, FIG. 4 is a bottom view thereof. The rear elevation view is the same as the front elevation view. The opposite side elevation view is the same as the side shown. The writing instrument depicted by means of broken lines in FIGS. 1 and 2 has been shown for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for a pen cap, as shown and described. 